Grounded Conductor Problems

[grandpapastu]

Problem:
120/240VAC 200A Single Phase residential service.
Completely severed and disconnected neutral at utility pole supplying the overhead service drop.
Phase conductors energized.
Question:
Would a main service disconnect GFCI circuit breaker interupt and disconnect the service to the house in the case of a disconnected or loose neutral?

On further review I conclude no. (Help)?
Also, I do not fully understand technical design and workings of a TWO POLE GFCI CB.

First thought is:
since I do not believe that a GFCI CB will do the trick, I need a Neutral Fault function available in the main CB whether the main disconnect CB is a GFCI CB or not.

 

[petersonra]

Re: Grounded Conductor Problems

a 2-pole gfci works exactly like a single pole unit. it compares the current in the two legs (either L and N or L1 and L2) and trips if they are not the same.

I don't see that any gfci would trip on an open neutral as it would not cause an imbalance in current between the two legs.

 

[grandpapastu]

Re: Grounded Conductor Problems

O.K. Bob but:
with the two pole GFCI the load neutral does not tap in series through the GFCI device to the grounding buss as it does with the single pole unit.

 

[al hildenbrand]

Re: Grounded Conductor Problems

This intrigues me.

There is a key piece of this: 120/240 V 200 A residential service (therefore it is single phase).

The GFCI is at the service.

So's the main bonding point between the neutral and the GES.

Where the GFCI measures the neutral current is going to have to be on the load side of the main bonding jumper for the GFCI to hold at all in the first place. As a result, the GFCI, by itself, won't "see" upstream towards the PoCo to determine how the current splits at the main bonding jumper. The GFCI will only watch that the currents out to the load(s) and returned from the load(s) sum to zero.

So, grandpapastu, the whole service GFCI will not be a neutral loss safety.

 

[al hildenbrand]

Re: Grounded Conductor Problems

grandpapastu,

Since the GFCI has to watch the neutral on the load side of the main bonding jumper, the (as you say) load neutral does tap in series through the GFCI device to the grounding buss.

 

[grandpapastu]

Re: Grounded Conductor Problems

Please send diagram of two pole GFCI CB design. (personal interest for research)
Two pole GFCI CB neutral tap conductor taps to the grounding buss and the load neutral does not feed through the device as it does with the single pole units.
Thank you all for the conference. I am now even more sure that the GFCI CB will not protect from this problem. I am POSITIVE, but I will still yet like to see the design requested.
I want a Neutral Fault CB. Now.
Such a very simple request?

 

[iwire]

Re: Grounded Conductor Problems

Al, grandpapastu had come up with an idea for a neutral loss detector.

I had suggested that a GFCI main would do the job.

I feel it would work fine as long as the GFCI main was located on the utility side of the bonding jumper.

If the neutral opened on the utility side the current 'taken' by the GES would create the imbalance needed to trip the GFCI.

Or am I missing something?

 

[al hildenbrand]

Re: Grounded Conductor Problems

Bob, the GES will be another path that the normal residence load unbalance current will travel in back to the PoCo transformer even when the neutral on the PoCo side of the service disconnect is intact.

The resistance of the GES is the unknown. With the GES in it's simplest form being a single ground rod (and arguably the highest resistance), all the way to a connection back to the PoCo supply through a residential domestic water supply delivered in metal water pipes and back through the nieghbor(s) GES and neutral(s) (probably the lowest resistance, maybe even lower than the service entrance neutral in question), the amount of current not in the service entrance conductors is unknown.

The neutral loss indicator would need to react to the change of the current split at the main bonding jumper, not the amount of the change, seems to me.

 

[iwire]

Re: Grounded Conductor Problems

Originally posted by al hildenbrand:
Bob, the GES will be another path that the normal residence load unbalance current will travel in back to the PoCo transformer even when the neutral on the PoCo side of the service disconnect is intact.

But it would not be a path through the GFCI.

 

[hurk27]

Re: Grounded Conductor Problems

A two pole GFCI breaker has all current carrying conductors pass through the current coil in it. (L,L,N) if all current is returned through this current coil on any of these conductors the coil will not develop any current to cause the unit to trip, even if the neutral is lost all current will still be from line to line and cancel each other out. But if there any connection between the grounded conductor and the grounding conductor on the load side of the GFCI the GFCI will trip immediately and not reset. The only way I could see this being done is to have a relay that would be placed between the grounded conductor and an isolated ground rod, it would have to have a coil load of less then .5 amps and a voltage rating of 50 to 120 volts to be able to detect the voltage to earth from a loss neutral this relay could be used to trip a shunt trip breaker. or trigger a GFP breaker to trip by providing a path to the isolated Earth rod to open the GFP.
Will it trip fast enough to protect the electronics in the building? I would say it would depend upon how balanced the panel is and how much current can pass back to the source via the grounding electrode system.

My summary is a two-pole GFCI main breaker will do nothing in the event that a neutral opens this is because the MBJ has to be on the line side of the GFCI before it will even stay on and that all the load current will still flow back to the source through the current transformer in the GFCI breaker.
Now put a relay in between the neutral and an isolated Earth rod and you will have a detection circuit. The coil could be as low as 12 volts as it should open the GFCI breaker fast enough to prevent it from receiving the full voltage drop of the lost neutral?

 

[iwire]

Re: Grounded Conductor Problems

Yeah I missed the boat on this one.

Right after my last post I went to the 'thinking room' and realized what Al was saying.

It can't work because it will trip immediately anyway.

 

[al hildenbrand]

Re: Grounded Conductor Problems

Bob,

But it would not be a path through the GFCI.

I agree.

Consider this hookup at House #3 in my diagram.

A GFCI needs L1 + L2 + N = Zero (or within a few milliamps of zero) for the GFCI not to trip.

A GFCI on the line side of the main bonding jumper at House #3 will see L1 + L2 + N = maybe 50 A or less. . .The GFCI won't hold under normal conditions.

Edit a small math error - Al

[ July 15, 2005, 07:11 PM: Message edited by: al hildenbrand ]

 

[al hildenbrand]

Re: Grounded Conductor Problems

Tag!

 

[pierre]

Re: Grounded Conductor Problems

If a 2 pole GFCI has a lost neutral, I do not think it will work regardless. Doesn't it need the neutral for the electronics of the unit to function?

 

[al hildenbrand]

Re: Grounded Conductor Problems

Pierre,

Yes, you're right.

Even if the load is straight line to line with no neutral, the GFI breaker has to have a connection to the neutral bus for the device to work.

 

[al hildenbrand]

Re: Grounded Conductor Problems

Seems to me we need a device that will respond to the change of the ratio of the currents in the main bonding jumper and the service entrance neutral.

 

[al hildenbrand]

Re: Grounded Conductor Problems

While a voltage relay might pick up some situations, I don't think it would have a chance in the situation in my diagram above.

The occupants of house #3 simply wouldn't notice the loss of the PoCo neutral, unless the resistance of the conductive path through the water system started increasing, causing voltage swings.

 

[hurk27]

Re: Grounded Conductor Problems

AL how about a current relay on the GEC that could be field set to trip if above a certain value. I been wanting to see if I could get one of the major manufactures to produce a simple current relay that would be able to prevent two appliance from running at the same time to allow more diversity when on a generator.

 

[George Stolz]

Re: Grounded Conductor Problems

Given the world as it is right now, I believe grandpapastu is on the right track. A special kind of device would be required to detect an open neutral to the service.

For a GFI to work, as mentioned, the bonding jumper would have to be gone, meaning that a four-wire supply would be necessary from the POCO. Given that none do this, it's moot.

A device that would register an open neutral at a service and disconnect the power would be very profitable for the installer, and the manufacturer, and advantageous for the customer. (Probably more so than lightning arrestors, IMO.)

A device at the service that placed a small uneven load on each phase, creating the recipe for a open neutral's destructive potential, with an IC powered independently by a capacitor monitoring line voltage would do the trick. It could then trigger a trip mechanism in the main, as you suggested.

Great theory, how do you build one?

 

[al hildenbrand]

Re: Grounded Conductor Problems

Wayne,

Setting a trip level based on the amount of current in the GEC will run into a problem when the running load in the house is light (say the middle of the night) and the PoCo neutral goes down.

If the trip level is set based on the GEC current level under higher load conditions, the neutral loss device won't respond.